A casual photographer wont buy this and a professional photographer probably got a dslr, I would consider bying it if it was like 10$
I've seen people say this a lot (MKBHD said this in his review for the Xperia phone) but I think it's a somewhat silly point to make. Yes, pros have DSLRs but you don't always have it with you as it's a huge, heavy thing.
You always have your small, lightweight phone on you and it has some very good camera hardware. Snapping a standard JPEG on an iPhone produces a pretty decent picture but if you want to create something incredible you need manual controls or RAW at the very least -- Halide gives you that power, it can be the difference between a decent looking 'quick snap' of something vs. a picture that could be framed.
Combine RAW capability with the plethora of desktop class editing software for iPhone and suddenly you've got yourself a mobile photography platform that has the best portability to quality ratio of any camera platform in existence right now. In fact most photographers I know say they love using their iPhone to take pictures in lieu of their DSLR setups simply because of its 'always there, always ready' status.
We don't even need to get into video capability but needless to say Filmic Pro + LumaFusion makes for better looking video than even some dedicated cameras for sale right now for twice the price.
$30 is nothing for a professional looking to push their mobile photography capabilities to the limit and it's a relatively steep but acceptable price for casual photographers looking to do the same (like me). The results speak for themselves, here's a couple of pics that won Halide's photo competition (I don't think I could capture these using the iPhone's built in app, do you?):
This one was taken with Spectre, the long exposure app created by the Halide team: